Andes Project 1

09-12-2007

The
Andes Project

Abstract:

The Western Escarpment of
the Andes of Northern Chile is characterized by the presence of gently
folded surfaces (or pediplains) tens to hundreds of km2 wide, and
>1500 m deep valleys that dissect these surfaces. The most prominent
structure is the Oxaya Anticline, dissected by the Lluta- and Azapa Valley.
We will argue that the formation of this anticline was enhanced due to
fluvial incision.

The Oxaya Anticline is
formed by Mesozoic basement and a Tertiary series of fluvio-volcanoclastics.
These Tertiary sediments cover a pre-existing relief defined by the top of
the Mesozoic. The fold axis of the anticline corresponds with today’s
maximum elevation of the unconformity between the Mesozoic basement and the
Tertiary unit, which clearly reflects the anticline geometry. The spatial
distribution of convergent and extensional structures with minor
displacement and the sub parallel strike-orientation of the anticline to the
Andes suggest buckling as predominant tectonic process. The anticline
developed supposedly as a response to crustal shortening in the plate
overriding the Farallon–Nazca plate, between the trench and the Western
Cordillera.

Cross-cutting relationships
between the Oxaya Anticline and dated sediments imply that its formation was
initiated by crustal shortening before at least 25 Ma in the Mesozoic
basement. At this time up to 1500 m of coarse-grained sediments started to
accumulate over some 40 km E–W distance between the Coastal Cordillera and
the western slope of the Andes. Further significant growth of the anticline
occurred later than 20 Ma (age of Oxaya-Formation, which is open folded).
The maximum growth, however, was supposedly reached at ca. 8 Ma as indicated
by the Lluta collapse (one of the largest paleo-landslides in the world),
and the inversion of the drainage system at the eastern flanks of the Oxaya
Anticline. This period of enhanced buckling coincides with the time
interval, when rates of down-cutting in the Lluta- and Azapa-valleys started
to accelerate.

The
conclusion is that the incision of these valleys potentially has had a major
influence on deformation of the surrounding Oxaya Anticline. Theoretical
models indicate that this is the case if incision occurs at the same time
the crust is deforming plastically in response to regional compression. In
this case, incision amplifies background deformation at relatively small
scale leading to the formation of non-cylindrical folds with culminations
coinciding with river incision, which, in turn, is consistent with the
geological data.